1. Field of the Invention
The present invention relates generally to recuperators for gas turbine engines and, more particularly, to a technique for maintaining substantially constant the pressure differential between inlet and outlet conduits which extend through a recuperator.
2. Discussion of the Prior Art
Brayton cycle engines generally comprise means, specifically, compressors, for compressing air for the support of combustion, a combustion chamber which has inlets for both the compressed air and fuel, and means for extracting energy from the hot exhaust gases to produce mechanical work. When a turbine is used to extract energy, the hot exhaust gases produced in the combustion chamber are fed to a turbine that rotates a driveshaft. In a recuperated turbine engine, exhaust gases of the turbine are passed through a recuperative heat exchanger that heats the relatively cold compressed air from the compressor to maximize the efficiency of the engine. The purpose of the recuperator, or regenerator as it is sometimes called when it is of a rotating construction, is to return some of the heat energy that would normally be lost with the exhaust, to the front of the combustion chamber. By doing this, less fuel needs to be added to reach the turbine limiting temperatures and this will result in high thermal efficiency, low specific fuel consumption, and low exhaust gas temperature. Recuperators are commonly used on ground-power engines, but to a lesser extent on aircraft engines since this method of power recovery often results in excessive weight and/or air-sealing difficulties.
Typical of current recuperator designs is the construction disclosed in U.S. Pat. No. 5,004,044 to Horgan et al. In that instance, an annular heat exchange apparatus is provided for radially conducting a first fluid from a center aperture to an outer perimeter and is adapted for conducting a second fluid through the apparatus. The apparatus comprises a plurality of heat exchange modules and a plurality of second fluid conduit members. The heat exchange modules each have a rectilinear heat exchange means with a first fluid inlet side at the center aperture. The first fluid inlet sides substantially define the center aperture. The plurality of second fluid conduit members are located between adjacent modules for conducting the second fluid into the modules.
Another typical construction is that disclosed in U.S. Pat. No. 4,474,000 to Benson et al which proposes unique seals as a solution to the problem of leakage between the hot exhaust and cold high pressure air sides of the heat exchanger or recuperator.
Also known to the prior art are a variety of constructions for controlling the flow of air through a heat exchanger. These include U.S. Pat. Nos. 4,971,768 to Ealba et al; 4,881,596 to Bergmann et al; 4,727,907 to Duncan; and 4,573,526 to Jung.
Customarily, in the design of gas turbine engines, it is desirable for the air flow velocity to be minimized as it exits the compressor and, in this manner, minimize pressure losses, for example, frictional losses, turning losses, and the like. However, it has more recently come to be realized that this approach often results in a penalty in terms of reduced recuperator performance and, possibly, even increased pressure losses. In a highly successful modern gas turbine engine, for example, the air flow into the recuperator plates is typically non-uniform such that the plate pairs located near the front header receive more air than those located close to the rear header. This maldistribution results in increased metal temperatures, and increased pressure drop, perhaps 10%-15%, above theoretical, and a reduced effectiveness, about 2%-3%.